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Abstract:

A current-amplifying transistor device is provided, between an emitter
electrode and a collector electrode, with two organic semiconductor
layers and a sheet-shaped base electrode. One of the organic
semiconductor layers is arranged between the emitter electrode and the
base collector electrode, and has a diode structure of a p-type organic
semiconductor layer and an n-type p-type organic semiconductor layer. A
current-amplifying, light-emitting transistor device including the
current-amplifying transistor device and an organic EL device portion
formed in the current-amplifying transistor device is also disclosed.

Claims:

1. A current-amplifying transistor device provided with an emitter
electrode, a collector electrode, first and second organic semiconductor
layers formed between the emitter electrode and the collector electrode,
and a sheet-shaped base electrode formed between the first organic
semiconductor layer and the second organic semiconductor layer, wherein
the first organic semiconductor layer is arranged between the emitter
electrode and the base electrode, and has a diode structure of a p-type
organic semiconductor layer and an n-type organic semiconductor layer.

2. The current-amplifying transistor device according to claim 1, wherein
the second organic semiconductor layer comprises an n-type organic
semiconductor layer arranged between the collector electrode and the base
electrode, and the first organic semiconductor layer has a diode
structure of a p-type organic semiconductor layer formed above the base
electrode and an n-type organic semiconductor layer formed below the
emitter electrode.

3. The current-amplifying transistor device according to claim 2, wherein
the p-type organic semiconductor layer and n-type organic semiconductor
layer in the first organic semiconductor layer are a hole transport
material layer and an electron transport material layer, respectively.

4. The current-amplifying transistor device according to claim 2, wherein
the p-type organic semiconductor layer in the first organic semiconductor
layer is formed from a metal phthalocyanine or a non-metal
phthalocyanine.

5. The current-amplifying transistor device according to claim 2, wherein
the p-type organic semiconductor layer in the first organic semiconductor
layer is formed from pentacene.

6. The current-amplifying transistor device according to claim 1, wherein
the second organic semiconductor layer is arranged between the collector
electrode and the base electrode, and comprises a stacked organic
semiconductor layer obtainable by stacking an organic semiconductor
layer, which is formed from N,N'-dimethylperylenetetracarboxylic acid
diimide (Me-PTCDI), and another organic semiconductor layer, which is
formed from fullerene (C60), together.

7. The current-amplifying transistor device according to claim 1, further
comprising a lithium fluoride layer formed between the base electrode and
the second organic semiconductor layer.

8. The current-amplifying transistor device according to claim 1, which
has a current amplification factor of at least 50 at a low voltage not
higher than 5 V.

9. The current-amplifying transistor device according to claim 1, which
has an on/off ratio of at least 100.

10. A current-amplifying, light-emitting transistor device comprising the
current-amplifying transistor device according to claim 1, and an organic
EL device portion formed between the p-type organic semiconductor layer
and the n-type organic semiconductor layer in the first organic
semiconductor layer, wherein the organic EL device portion comprises an
organic light-emitting layer and at least one layer selected from the
group consisting of a hole injection layer, a hole transport layer, an
electron transport layer and an electron injection layer.

11. A current-amplifying, light-emitting transistor device comprising the
current-amplifying transistor device according to claim 1 and an organic
EL device portion formed between the second organic semiconductor layer
and the collector electrode, wherein the organic EL device portion
comprises an organic light-emitting layer and at least one layer selected
from the group consisting of a hole injection layer, a hole transport
layer, an electron transport layer and an electron injection layer.

12. The current-amplifying transistor device according to claim 2,
wherein the second organic semiconductor layer is arranged between the
collector electrode and the base electrode, and comprises a stacked
organic semiconductor layer obtainable by stacking an organic
semiconductor layer, which is formed from
N,N'-dimethylperylenetetracarboxylic acid diimide (Me-PTCDI), and another
organic semiconductor layer, which is formed from fullerene (C60),
together.

13. The current-amplifying transistor device according to claim 3,
wherein the second organic semiconductor layer is arranged between the
collector electrode and the base electrode, and comprises a stacked
organic semiconductor layer obtainable by stacking an organic
semiconductor layer, which is formed from
N,N'-dimethylperylenetetracarboxylic acid diimide (Me-PTCDI), and another
organic semiconductor layer, which is formed from fullerene (C60),
together.

14. The current-amplifying transistor device according to claim 4,
wherein the second organic semiconductor layer is arranged between the
collector electrode and the base electrode, and comprises a stacked
organic semiconductor layer obtainable by stacking an organic
semiconductor layer, which is formed from
N,N'-dimethylperylenetetracarboxylic acid diimide (Me-PTCDI), and another
organic semiconductor layer, which is formed from fullerene (C60),
together.

15. The current-amplifying transistor device according to claim 5,
wherein the second organic semiconductor layer is arranged between the
collector electrode and the base electrode, and comprises a stacked
organic semiconductor layer obtainable by stacking an organic
semiconductor layer, which is formed from
N,N'-dimethylperylenetetracarboxylic acid diimide (Me-PTCDI), and another
organic semiconductor layer, which is formed from fullerene (C60),
together.

16. The current-amplifying transistor device according to claim 2,
further comprising a lithium fluoride layer formed between the base
electrode and the second organic semiconductor layer.

17. The current-amplifying transistor device according to claim 3,
further comprising a lithium fluoride layer formed between the base
electrode and the second organic semiconductor layer.

18. The current-amplifying transistor device according to claim 4,
further comprising a lithium fluoride layer formed between the base
electrode and the second organic semiconductor layer.

19. The current-amplifying transistor device according to claim 5,
further comprising a lithium fluoride layer formed between the base
electrode and the second organic semiconductor layer.

20. The current-amplifying transistor device according to claim 6,
further comprising a lithium fluoride layer formed between the base
electrode and the second organic semiconductor layer.

Description:

TECHNICAL FIELD

[0001] This invention relates to a current-amplifying transistor device
and a current-amplifying, light-emitting transistor device, each of which
has current-amplifying capability, and more specifically, to a
current-amplifying transistor device and a current-amplifying,
light-emitting transistor device, each of which can perform large-current
modulation at a low voltage and is superb in on/off ratio, and therefore,
is excellent in driving an organic EL display or the like.

BACKGROUND ART

[0002] In recent years, flat-screen TV sets and notebook-size personal
computers have come into wide use, resulting in increasing demands for
liquid crystal displays, organic EL displays, electronic paper displays,
and the like. For driving devices in these displays, field-effect
transistors (FETs) are used. FETs making use of silicon, an inorganic
material, are primarily used these days, but reports have been made about
displays that use organic transistor devices for lower manufacturing
cost, larger screen size and flexibilization.

[0003] Most of such displays, however, rely upon a combination of organic
field-effect transistors (OFETs) and liquid crystal or electrophoretic
cells. OFET can hardly provide a large current due to its structure and
low mobility. Practically no report has hence been made about a case in
which OFETs are used as drive devices in an organic EL display, said
drive devices being current-driven devices that require a large current.
There is, accordingly, an outstanding desire for the development of
organic transistor devices that can output a large current at a low
voltage and can drive an organic EL display.

[0004] For obtaining a large current with OFET, its channel length needs
to be shortened at present. With a patterning technology developed with a
view to mass production, however, it is difficult to shorten the channel
length to several micrometers or less. To resolve this problem, research
is under way about "vertical-type transistor structures" that can obtain
a large current in a low voltage range by causing a current to flow in
the thickness direction of films. Film thicknesses generally employed in
vertical-type transistors range from several tens nanometers to several
hundreds nanometers, and can be controlled with high accuracy on the
order of several angstroms. The fabrication of a vertical-type transistor
with its channel extending in the film thickness direction (vertical
direction) has a possibility that a channel length of 1 μm or less may
be easily realized to obtain a large current. As such vertical-type
organic transistor devices known to date, there are vertical-type
transistors of a polymer-grid triode structure, in each of which a
self-organizing network structure in the form of a polyaniline film is
used as a grid electrode, and static induction transistors (SITs) in each
of which a source-drain current is controlled by modulating the width of
a depletion layer in a microstripe-shaped intermediate electrode.

[0005] Recently, there has been proposed a vertical-type organic
transistor device that has a stacked structure of organic
semiconductor/metal/organic semiconductor and can show high-performance
transistor characteristics (PTL 1). This vertical-type transistor device
is provided, between an emitter electrode and a collector electrode, with
organic semiconductor layers and a stripe-shaped intermediate metal
electrode. Upon passage of electrons, which have been injected from the
emitter electrode, through the intermediate metal electrode in the
organic transistor device, current amplification similar to that
available from a bipolar transistor is observed, and the intermediate
metal electrode serves like a base electrode. The vertical-type
transistor device is, therefore, called a "metal-base organic transistor"
(which may hereinafter be called "MBOT").

[0006] In MBOT, no substantial current flows when an output voltage is
applied between the emitter electrode and the collector electrode and no
voltage is applied between the emitter electrode and the base electrode,
but a current flows between the emitter electrode and the collector
electrode when a voltage is applied between the emitter electrode and the
base electrode. The current that flows between the emitter electrode and
the collector electrode is a collector current, while the current that
flows between the emitter electrode and the base electrode is a base
current. Compared with the base current that increases upon application
of a base voltage, the collector current rapidly increases. MBOT,
therefore, can serve as a device that can modulate the collector current
by the base voltage. A "leakage current" that may happen to flow when a
voltage is applied between the emitter electrode and the collector
electrode but no voltage is applied between the emitter electrode and the
base electrode is an off-state current, while a current that flows upon
application of a voltage between the emitter electrode and the base
electrode is an on-state current. MBOT is a transistor device that allows
substantially no off-state current and can provide a large on-state
current.

[0007] As the structure of an organic transistor (MBOT), there has been
reported MBOT that can be readily fabricated by providing a transparent
ITO electrode as a collector electrode and stacking organic
semiconductor/metal/organic semiconductor on the transparent ITO
electrode by vacuum deposition (PTL 2). Employed as the organic
semiconductors are N,N'-dimethylperylenetetracarboxylic acid diimide
(Me-PTCDI) and fullerene (C60), which are n-type organic semiconductor
materials. Employed as electrode materials, on the other hand, are Al for
a base electrode and Ag for an emitter electrode. By introducing a
dark-current suppression layer and subjecting the base electrode to heat
treatment, this MBOT serves as a transistor device that can perform
large-current amplification with an improved on/off ratio (ratio of
on-state current to off-state current). As is appreciated from the
foregoing, MBOT is characterized in that it does not need micropatterning
for a microgrid-shaped electrode or microstripe-shaped electrode although
it is a vertical-type transistor.

[0008] Also reported are organic transistor devices (MBOTs), each of which
is reported to provide good current amplification characteristics and
on/off ratio without applying heat treatment or the like. They are MBOTs
each having organic semiconductor layers and a sheet-shaped base
electrode between an emitter electrode and a collector electrode and also
having an energy barrier layer and a charge-pass promoting layer between
the base electrode and the collector electrode (PTL 3); and MBOT making
uses, as a collector layer, of an organic semiconductor layer formed from
a perylenetetracarboxylic acid diimide having long-chain alkyl groups
(Japanese Patent Application 2009-114619).

[0009] As a vertical-type transistor, an organic transistor provided with
a light-transmitting metal substrate has also been reported as a bipolar
transistor. This organic transistor has organic semiconductor layers and
a sheet-shaped base electrode between an emitter electrode and a
collector electrode, and as the organic semiconductor layers, one being
between the emitter electrode and the base electrode and the other
between the collector electrode and the base electrode, uses
hetero-junction organic semiconductor layers formed from
N,N'-diphenyl-N,N'-di(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPD) and
fullerene (C60), respectively (NPL 1).

[0014] It is, however, difficult to mass-fabricate vertical-type
transistors of the polymer-grid triode structure or static induction
transistors (SITs) with higher performance because of the difficulty in
forming intermediate electrodes. The organic transistor devices (MBOTs)
disclosed in Patent Citations 1 and 2 may provide a high off-state
current depending on the film thickness and structure, and moreover, a
current-amplifying effect is not necessarily observed on the
above-described transistors even when a stacked structure of organic
semiconductor/metal/organic semiconductor is formed. To show stable
performance and to obtain a large current value, a high current
amplification factor and a high on/off ratio, it is necessary to form an
oxide layer, as an off-state current suppression layer, on a surface of a
base electrode by heat treatment.

[0015] On the other hand, the organic transistor devices (MBOTs) disclosed
in Patent Citation 3 and the earlier application (Japanese Patent
Application 2009-114619) can amplify a current without applying heat
treatment to an electrode for the formation of an off-state current
suppression layer. They are, however, difficult to provide a large
current value, large current amplification factor and high on/off ratio,
which are sufficient to operate electronic equipment.

[0016] The organic transistor disclosed in NPL 1 and provided with the
light-transmitting metal substrate may find utility in a complementary
logic circuit or the like as it shows a current modulating effect as a
bipolar transistor device. However, it has difficulty in providing a
large current value, in other words, in increasing a current to such an
extent as to operate electronic equipment, and therefore, can be hardly
employed as a drive device in an organic EL display or the like.

[0017] The present invention has been made to resolve the above-described
problem, and has as an object thereof the provision of a metal-base
organic transistor device (MBOT), which exhibits a large
current-amplifying effect at a low voltage between an emitter electrode
and a collector electrode and is excellent in on/off ratio, by a simple
fabrication process that does not require a treatment step such as
heating. Another object of the present invention is to provide a
light-emitting transistor device, which has an organic EL device portion
including a light-emitting layer, can provide an excellent on/off ratio
and a high current density, and can perform a self-emission.

Solution to Problem

[0018] The above-described objects can be achieved by the present
invention to be described hereinafter. Described specifically, in one
aspect of the present invention, there is provided a current-amplifying
transistor device provided with an emitter electrode, a collector
electrode, first and second organic semiconductor layers formed between
the emitter electrode and collector electrode, and a sheet-shaped base
electrode formed between the first organic semiconductor layer and the
second organic semiconductor layer, wherein the first organic
semiconductor layer is arranged between the emitter electrode and the
base electrode, and has a diode structure of a p-type organic
semiconductor layer and an n-type organic semiconductor layer. Owing to
the adoption of the diode structure, the current-amplifying transistor
device is excellent in on/off ratio, and can perform large-current
amplification.

[0019] Preferably, the second organic semiconductor layer may comprise an
n-type organic semiconductor layer arranged between the collector
electrode and the base electrode, and the first organic semiconductor
layer may have a diode structure of a p-type organic semiconductor layer
formed above the base electrode and an n-type organic semiconductor layer
formed below the emitter electrode.

[0020] The p-type organic semiconductor layer and n-type organic
semiconductor layer in the first organic semiconductor layer may be, for
example, a hole transport material layer and an electron transport
material layer, respectively. The p-type organic semiconductor layer in
the first organic semiconductor layer may be formed, for example, from a
metal phthalocyanine or a non-metal phthalocyanine or from pentacene.

[0021] For example, the second organic semiconductor layer may be arranged
between the collector electrode and the base electrode, and may comprise
a stacked organic semiconductor layer obtainable by stacking an organic
semiconductor layer, which is formed from
N,N'-dimethylperylenetetracarboxylic acid diimide (Me-PTCDI), and another
organic semiconductor layer, which is formed from fullerene (C60),
together.

[0022] Preferably, the current-amplifying transistor device may further
comprise a lithium fluoride layer formed between the base electrode and
the second organic semiconductor layer.

[0023] The current-amplifying transistor device may have, for example, a
current amplification factor of at least 50 at a low voltage not higher
than 5 V. Further, the current-amplifying transistor device may have, for
example, an on/off ratio of at least 100.

[0024] In another aspect of the present invention, there is also provided
a current-amplifying, light-emitting transistor device comprising the
above-described current-amplifying transistor device and an organic EL
device portion formed between the p-type organic semiconductor layer and
the n-type organic semiconductor layer in the first organic semiconductor
layer, wherein the organic EL device portion comprises an organic
light-emitting layer and at least one layer selected from the group
consisting of a hole injection layer, a hole transport layer, an electron
transport layer and an electron injection layer; or a current-amplifying,
light-emitting transistor device comprising the above-described
current-amplifying transistor device and an organic EL device portion
formed between the second organic semiconductor layer and the collector
electrode, wherein the organic EL device portion comprises an organic
light-emitting layer and at least one layer selected from the group
consisting of a hole injection layer, a hole transport layer, an electron
transport layer and an electron injection layer.

ADVANTAGEOUS EFFECTS OF INVENTION

[0025] Owing to the inclusion of the diode structure in one of the organic
semiconductor layers, the current-amplifying transistor device and
current-amplifying, light-emitting transistor device according to the
present invention can stably provide a current-amplifying effect that
enables to perform, at a low voltage, amplification into a large current.

[0026] The current-amplifying transistor device (MBOT) according to the
present invention is useful as a drive device for various displays,
especially as a drive device for organic EL displays and electronic paper
displays which are driven by large-current modulation. Transistor devices
for driving these displays are needed to provide a high contrast between
an off-state and an on-state so that a higher on/off ratio and
suppression of a dark current are required. A low on/off ratio and a
large dark current cause a problem such that an organic EL display may
emit light even in an off-state. The current-amplifying transistor device
according to the present invention is high in on/off ratio and is
excellent in large-current modulation characteristics and frequency
characteristics in a low voltage range, and therefore, can have high
performance as a drive transistor device.

[0027] Further, the current-amplifying transistor device according to the
present invention can perform large-current modulation in a low voltage
range, can reduce the area occupied by a transistor device in a single
pixel, and can provide a display with an improved aperture rate. As a
consequence, a high-performance, high-efficiency display can be provided.
The current-amplifying transistor device according to the present
invention can be fabricated by a vapor deposition process. By forming
such current-amplifying transistor devices on a flexible substrate of
plastics or the like, a small-size, light-weight display or equipment can
be manufactured.

[0028] The current-amplifying, light-emitting transistor device according
to the present invention, which has the organic EL device portion formed
between the p-type organic semiconductor layer and the n-type organic
semiconductor layer in the first organic semiconductor layer or between
the second organic semiconductor layer and the collector layer, can be
fabricated in a form that an organic light-emitting device and its drive
transistor device are combined into a single device. As the organic EL
device portion has the organic light-emitting layer, a surface light
emission is feasible from the electrode surfaces of such transistor
devices. Moreover, no micropatterning is needed for the base electrode
unlike the conventional SIT structure, large-current modulation is
feasible in a low voltage range, and a high on/off ratio is available.
The current-amplifying, light-emitting transistor device is, therefore,
excellent as a light-emitting transistor device. Because the
current-amplifying, light-emitting transistor device can be fabricated by
a vapor deposition process only, the device can be formed on a flexible
substrate of plastics or the like. The current-amplifying, light-emitting
transistor device can, therefore, be provided as a current-amplifying,
light-emitting transistor device of a small-size, light-weight and simple
structure. The current-amplifying transistor device and
current-amplifying, light-emitting transistor device may hereinafter be
called simply "transistor device and light-emitting transistor device" or
"MBOT and light-emitting MBOT".

BRIEF DESCRIPTION OF DRAWINGS

[0029] FIG. 1 is a simplified schematic cross-sectional view illustrating
the construction of a transistor device according to one embodiment of
the present invention and its drive circuit.

[0030] FIG. 2 is a simplified schematic cross-sectional view illustrating
the construction of a light-emitting transistor device A according to
another embodiment of the present invention and its drive circuit.

[0031]FIG. 3 is a simplified schematic cross-sectional view illustrating
the construction of a light-emitting transistor device B according to a
further embodiment of the present invention and its drive circuit.

[0032] FIG. 4 is a simplified schematic cross-sectional view illustrating
the construction of an EL device portion in the light-emitting transistor
device A or B.

[0033] FIG. 5 is a simplified schematic cross-sectional view illustrating
the construction of a transistor device fabricated in Example 1.

[0034]FIG. 6 is a simplified schematic cross-sectional view illustrating
the construction of a transistor device fabricated in Example 3.

[0035] FIG. 7 is a diagram illustrating output characteristics (Ic-Vb
curves) of the transistor device of Example 1 and a transistor device of
Comparative Example 1.

[0036] FIG. 8 is a diagram illustrating output characteristics (Ic-Vb
curves) of a transistor device of Example 2 and the transistor device of
Comparative Example 1.

DESCRIPTION OF EMBODIMENTS

[0037] The present invention will hereinafter be described in detail based
on certain embodiments of the present invention. It should, however, be
borne in mind that the present invention shall not be limited to or by
the following embodiments.

[0038] A description will first be made about a current-amplifying
transistor device (MBOT) according to the one embodiment of the present
invention, in which first and second organic semiconductor layers and a
sheet-shaped base electrode are arranged between an emitter electrode and
a collector electrode. As the first organic semiconductor layer, an
emitter layer having a diode structure is used.

[0039] The transistor device of this embodiment is a vertical-type,
metal-base, organic transistor device (MBOT), which as illustrated in
FIG. 1, has a stacked structure of organic
semiconductor/electrode/organic semiconductor and can be fabricated by a
simple stacking process. Its structure includes, in an ascending order, a
substrate (not shown in FIG. 1), a collector electrode 11, a collector
layer (the second organic semiconductor layer) 21 formed of an organic
semiconductor layer, a base electrode 13, an emitter layer (the first
organic semiconductor layer) 22(22A,22B) having a stacked diode
structure, and an emitter electrode 12.

[0040] For the fabrication of the transistor device of this embodiment,
the collector electrode 11 and collector layer 21 are successively formed
on the substrate, and further, the base electrode 13 and emitter layer
22(22A,22B) are stacked. Above the emitter layer 22, the emitter
electrode 12 is formed further to provide the transistor device of this
embodiment. The emitter layer 22 makes up a diode structure formed of an
organic semiconductor layer 22A and an organic semiconductor layer 22B
stacked together. Designated at signs 22A and 22B are specifically a
combination of an n-type organic semiconductor layer and a p-type organic
semiconductor layer. When the organic semiconductor layer 22A is the
n-type organic semiconductor layer, the organic semiconductor layer 22B
is formed as the p-type organic semiconductor layer. When the organic
semiconductor layer 22A is the p-type organic semiconductor layer, on the
other hand, the organic semiconductor layer 22B is formed as the n-type
organic semiconductor layer. As to the construction of the emitter layer
22, signs 22A,22B designate the n-type organic semiconductor layer and
the p-type organic semiconductor layer, respectively, when the collector
layer 21 is formed as an n-type organic semiconductor layer, or indicate
the p-type organic semiconductor layer and the n-type organic
semiconductor layer, respectively, when the collector layer 21 is formed
as a p-type organic semiconductor layer.

[0041] As the diode structure in the transistor device of this embodiment,
the emitter layer 22 has the diode structure formed of the p-type organic
semiconductor layer and the n-type organic semiconductor layer. Owing to
the effect of this diode structure, the transistor device of this
embodiment has effects to increase a base current and a collector current
and also to suppress a dark current and hence to maintain an off-state
current small. The transistor device of this embodiment is, therefore,
useful as an organic transistor device (MBOT) that can perform large
output modulation and large-current amplification in a low voltage range.

[0042] A description will next be made about a current that flows through
the transistor device of this embodiment. When a collector voltage Vc is
applied between the emitter electrode 12 and the collector electrode 11
and a base voltage Vb is applied between the emitter electrode 12 and the
base electrode 13, electrons injected from the emitter electrode 12 are
accelerated under the action of the base voltage, and therefore, pass
through the base electrode 13 and reach the collector electrode 11. In
other words, a base current Ib, which flows upon application of the base
voltage Vb between the emitter electrode 12 and the base electrode 13, is
amplified into a collector current Ic that by the application of the base
voltage, flows between the emitter electrode 12 and the collector
electrode 11. Therefore, the transistor device of this embodiment can
stably bring about a similar current-amplifying effect as a bipolar
transistor device, and can perform large output modulation and current
amplification.

[0043] For example, MBOT with a diode structure formed of an n-type
semiconductor layer (fullerene) and a p-type semiconductor layer (copper
phthalocyanine) can provide, as a current Ic, a large current
approximately 10 times or more compared with a current Ic available from
the use of a single n-type organic semiconductor (fullerene). Although
its principle is not clear, it may presumably be possible to consider the
following mechanism: [1] the current amplification factor is
significantly improved, [2] owing to the introduction of the
light-emitting layer in the layer of the diode structure, an emission of
light is feasible as a result of recombination of holes and electrons,
[3] because the off-state current does not increase, electrons flow from
around the base electrode toward the collector, a hole current is
produced toward the emitter layer, and the current pass rate through the
base electrode is also improved. Owing to the rectification effect of the
diode structure, the off-state current is suppressed, and a current of
electrons and a current of holes flow between the emitter electrode and
the base electrode with the diode layer interposed therebetween. As a
result, the diode structure is considered to enable larger current
amplification than the single layer.

[0044] Owing to the diode structure, the organic transistor device (MBOT)
of this embodiment can effectively suppress a leakage current, which is
not needed for the operation of the transistor device and may also be
called "an off-state current" or "a dark current" that flows in a
switch-off state, from flowing between the base electrode and the
collector electrode when no voltage Vb (Vb=0 V) is applied between the
emitter electrode and the base electrode. As a result, the organic
transistor device of this embodiment is provided with an improved on/off
ratio. When organic transistor devices (MBOT) are employed as drive
transistor devices in an organic EL display, large dark currents cause
emissions of light from the organic EL devices even when they are
switched off. These emissions of light lead to a reduction in the
contrast between an on-state and an off-state. It is, therefore, required
for a drive transistor device to have a high on/off ratio, preferably an
on/off ratio of 10 or higher, with an on/off ratio of 100 or higher being
more preferred.

[0045] The transistor device (MBOT) of this embodiment has the diode
structure between the emitter electrode and the base electrode. In an
off-state, substantially no current (off-state current) is, therefore,
allowed to flow from the base electrode to the emitter electrode owing to
the rectification effect of the diode structure. A dark current that
flows in an off-state is, therefore, suppressed so that a high on/off
ratio is obtained.

[0046] Even in a low voltage range, the transistor device (MBOT) of this
embodiment can show a large current amplification effect and can obtain a
large current. In general, an organic EL device is driven in a low
voltage range, so that its drive transistor device is required to output
a large current at several volts. If a voltage to be applied is set high,
an organic EL device can obtain a large current and can realize an
emission of high-intensity light. However, such a high voltage induces
deterioration or degradation of the materials of the organic EL device,
so that the organic EL device is shortened in service life and cannot
perform stable emissions of light over a long term. Therefore, the drive
voltage may range from 1 to 20 V, with 5 V or lower being preferred. The
current value to be obtained as a result of amplification by the
transistor device in this low voltage range may range preferably from 10
mA/cm2 to 500 mA/cm2, more preferably from 20 mA/cm2 to
200 mA/cm2. A current value smaller than 10 mA/cm2 cannot allow
the organic EL device to emit sufficient light, so that no sufficient
emission intensity can be obtained. A current value greater than 500
mA/cm2, on the other hand, cannot provide a sufficient on/off ratio,
leading to a potential problem that even in an off-state (voltage: 0 V),
a dark current may be produced and light may be emitted from the organic
EL device.

[0047] The emitter layer in the transistor device of this embodiment has
the diode structure of the p-type organic semiconductor layer and the
n-type organic semiconductor layer stacked together. Any material can be
used to form the emitter layer of the stacked structure insofar as the
emitter layer can function as a diode. A combination of a p-type organic
semiconductor material and an n-type organic semiconductor material can
be used without problem. The organic semiconductor material used in the
p-type organic semiconductor layer functions as a hole-transporting
semiconductor. Any material can be used without any particular limitation
insofar as it is a material that can transport holes (hole transport
material). On the other hand, the organic semiconductor material used in
the n-type organic semiconductor layer functions as an
electron-transporting semiconductor. Any material can be used without any
limitation insofar as it is a material that can transport electrons
(electron transport material).

[0048] The construction of the emitter layer in the transistor device of
this embodiment is determined depending on whether the collector layer
interposed between the base electrode and the collector electrode is an
n-type organic semiconductor layer or a p-type organic semiconductor
layer. When the collector layer is formed from an n-type organic
semiconductor material, the p-type semiconductor layer is formed on the
base electrode and the n-type semiconductor layer is formed below the
emitter electrode. When the collector layer is formed from a p-type
semiconductor material, on the other hand, an n-type semiconductor layer
is formed on the base electrode and a p-type semiconductor layer is
formed below the emitter electrode. As a particularly preferred
embodiment, it is possible to mention an organic transistor device (MBOT)
in which in view of the energy levels of the used electrodes and the HOMO
and LUMO energy levels of the organic semiconductor layers, a collector
layer is formed of an n-type organic semiconductor layer, a p-type
organic semiconductor layer is formed on the base electrode, and further,
an n-type organic semiconductor layer is stacked to form an emitter
electrode.

[0049] The current-amplifying, light-emitting transistor device according
to the present invention requires no micropatterning for the base
electrode unlike the conventional SIT structure, can perform
large-current modulation at a low voltage, and is high in on/off ratio.
Further, the current-amplifying, light-emitting transistor device can be
fabricated by a vapor deposition process only, can also be formed on a
flexible substrate of plastics or the like and has a small-size,
light-weight and simple structure, and therefore, is practical.

(Light-Emitting Transistor Device A)

[0050] By forming a light-emitting layer in the emitter layer of the
transistor device (MBOT) of this embodiment, an emission of light occurs
as a result of deactivation of excitons formed by recombination between
holes associated with a hole current from the base electrode and
electrons from the emitter electrode. The transistor device of this
embodiment can, therefore, be used as a light-emitting transistor device
A. Any light-emitting layer can be used without any problem insofar as it
is formed from a light-emitting material employed in organic EL devices.
The light-emitting layer can be formed between the p-type semiconductor
layer and the n-type semiconductor layer. When the n-type semiconductor
layer or p-type semiconductor layer is formed from a light-emitting,
organic semiconductor material, the organic semiconductor layer can also
serve as the light-emitting layer so that no additional light-emitting
layer needs to be formed. To obtain an improved luminous efficiency, an
electron injection layer, a hole injection layer and an exciton blocking
layer may be formed.

(Light-Emitting Transistor Device B)

[0051] A light-emitting transistor device B is provided when in the
transistor device of this embodiment, the collector layer has, as an
organic EL device portion, a light-emitting device portion including a
light-emitting layer and the light-emitting device portion comprises one
or more layers selected from a hole injection layer, a hole transport
layer, an electron transport layer and an electron injection layer.

[0052] A description will next be made about the structures and materials
of the individual elements in the transistor device of this embodiment.

(Substrate)

[0053] No particular limitation is imposed on the material of the
substrate that forms the organic transistor device of this embodiment,
insofar as it can retain the configuration of the transistor device.
Usable examples include inorganic materials such as glass, alumina,
silica and silicon carbide, metal materials such as aluminum, copper and
gold, and plastics such as polyimides, polyesters, polyethylene,
polystyrene, polypropylene, polycarbonates and polymethyl methacrylate.
The use of a plastic substrate makes it possible to fabricate a
transistor device which is light in weight, is excellent in impact
resistance, and is flexible. When desired to use the transistor device as
a light-emitting transistor device which has an organic light-emitting
layer formed therein and is of the bottom emission type that light is
released from the side of the substrate, the use of a substrate having
high optical transparency, such as a plastic film or glass substrate, is
desired. These substrates may be used either singly or in combination. No
particular limitation is on the size and form of the substrate insofar as
the formation of the transistor device is feasible. For example, a
substrate of any desired form such as a card, film, disk or chip can be
used without any problem.

(Organic Semiconductor Layer)

[0054] The organic semiconductor layer that forms the organic transistor
device of this embodiment is characterized in that as illustrated in FIG.
1, it comprises the collector layer 21 arranged between the collector
electrode 11 and the base electrode 13 and the emitter layer 22 formed
between the base electrode 13 and the emitter layer 12, and the emitter
layer 22(22A,22B) has the diode structure in the form of the stacked
structure of the n-type semiconductor layer and p-type semiconductor
layer.

(Emitter Layer)

[0055] As the emitter layer in this embodiment, the organic semiconductor
layer 22(22A,22B) arranged between the emitter electrode and the base
electrode has the diode structure formed of the p-type organic
semiconductor layer and n-type organic semiconductor. The construction of
the diode structure can be selectively determined depending on the
organic semiconductor materials to be used for the emitter layer and
collector layer, respectively. When the collector layer is formed from an
n-type organic semiconductor layer, the emitter layer may desirably be
formed by stacking the p-type organic semiconductor layer on the base
electrode and further, the n-type organic semiconductor layer on the
p-type organic semiconductor layer. When the collector layer is formed
from a p-type organic semiconductor layer, on the other hand, the emitter
layer may desirably be formed by stacking the n-type organic
semiconductor layer on the base electrode and further, the p-type organic
semiconductor layer on the n-type organic semiconductor layer.

[0056] The p-type organic semiconductor layer used in the emitter layer in
this embodiment has a function that it receives holes from the base
electrode or emitter electrode and transports them to its counterpart
n-type organic semiconductor layer or to a vicinity of its interface with
the its counterpart n-type organic semiconductor layer. Any desired
material can be used as a material for forming the p-type organic
semiconductor layer without any particular limitation insofar as it is a
common p-type semiconductor material. Usable examples include pentacene,
non-metal phthalocyanine, metal phthalocyanines (Cu-Pc, VO-Pc, Ni-Pc, and
so on), naphthalocyanine, indigo, thioindigo, anthracene, quinacridone,
oxadiazole, triphenylamine, triazole, imidazole, imidazolone, pyrazoline,
tetrahydroimidazole, polythiophene, porphyrin, and naphthothiophene, and
their derivatives. In addition to the p-type organic semiconductor
material, a hole transport material can also be used as a p-type organic
semiconductor material.

[0058] The p-type semiconductor material that forms the emitter layer in
this embodiment may preferably have electrical stability and adequate
ionization potential and electron affinity. Particularly preferred
materials include pentacene, and phthalocyanines such as copper
phthalocyanine and non-metal phthalocyanines, as p-type semiconductor
materials; and PDOT/PSS as a hole transport material.

[0059] Material other than those described above may also be used as the
p-type semiconductor material provided that they have higher transport
capability for holes than for electrons. The p-type semiconductor layer
may have not only a single layer structure making use of a single p-type
semiconductor material but also a mixed layer structure formed from two
or more p-type semiconductor materials or a stacked structure formed of
two or more organic semiconductor layers of different p-type
semiconductor materials. The p-type organic semiconductor layer can be
formed by a vapor deposition process or by one of various printing or
coating processes that make use of a solution or dispersion containing
the above-described p-type semiconductor material.

[0060] The n-type organic semiconductor layer employed in the emitter
layer in this embodiment has a function that it receives electrons from
the base electrode or emitter electrode and transports them to its
counterpart p-type organic semiconductor layer or to a vicinity of its
interface with the its counterpart p-type organic semiconductor layer.
The n-type organic semiconductor material that forms the n-type organic
semiconductor layer may preferably have electrical stability and adequate
ionization potential and electron affinity. Any desired material can be
used as a material for forming the n-type organic semiconductor layer
without any particular limitation insofar as it is a common n-type
semiconductor material. Examples of the n-type semiconductor material for
use in the emitter layer include Alq3
(tris(8-hydroxyquinolinol)aluminum) complex), naphthalene tetracarboxylic
anhydride (NTCDA), dialkylnaphthalenetetracarboxylic anhydride diimides
(NTCDI), perylene, perylenetetracarboxylic anhydride (PTCDA),
dialkylperylenetetracarboxylic diimides (PTCDI),
perylenebisbenzoimidazole (PTCBI), dialkylanthraquinones,
fluorenylidenemethane, tetracyanoethylene, fluorenone, diphenoquinone
oxadiazole, anthrone, thiopyran dioxide, diphenoquinone, benzoquinone,
malononitrile, dinitrobenzene, nitroanthraquinone, pyridine, pyrimidine,
maleic anhydride, pentacene fluoride, phthalocyanine fluoride,
alkyloligothiophene fluorides, fullerenes, carbon nanotube and carbon
nanohorn, and their derivatives. Particularly preferred materials include
fullerenes represented by C60 and perylenetetracarboxylic acid
derivatives represented by dimethylperylenetetracarboxylic diimide
(Me-PTCDI).

[0061] For the n-type organic semiconductor material for forming the
emitter layer in this embodiment, the parameter value that indicates its
ionization potential is generally important. Materials other than those
described above may also be used as the n-type semiconductor layer
provided that they have higher transport capability for holes than for
electrons. The n-type semiconductor layer may have not only a single
layer structure making use of a single n-type semiconductor material but
also a mixed layer structure formed from two or more n-type semiconductor
materials or a stacked structure formed of two or more organic
semiconductor layers of different n-type semiconductor materials. The
n-type organic semiconductor layer can be formed by a vapor deposition
process or by a coating process that makes use of a coating formulation
containing the above-described n-type semiconductor material.

[0062] Although the emitter layer in this embodiment can be formed by
using an n-type organic semiconductor material of a single chemical
structure and a p-type organic semiconductor material of a single
chemical structure, the n-type organic semiconductor material may be
mixed with an electron transport material of a different chemical
structure and the p-type organic semiconductor material may be mixed with
a hole transport material of a different chemical structure. These n-type
organic semiconductor material and p-type organic semiconductor material
are stacked to form an n-type organic semiconductor layer and a p-type
organic semiconductor layer joined together such that an emitter layer is
formed as a diode structure. As a particularly preferred combination of
materials, it is possible to mention fullerene (C60) as the n-type
organic semiconductor layer and copper phthalocyanine or pentacene as the
p-type organic semiconductor layer. The emitter layer 22 as an organic
semiconductor layer may desirably be formed by stacking the p-type
organic semiconductor layer 22A and the n-type organic semiconductor
layer 22B on the base electrode 13, although the emitter layer 22 can be
used without any particular problem insofar as it has a diode structure.

[0063] The organic semiconductor layer that makes up the emitter layer in
this embodiment may desirably have a high charge mobility, with at least
0.0001 cm2/Vs being preferred.

[0064] The thicknesses of the emitter layers 22A,22B may preferably be
smaller than the collector layer in principle, and can each be 300 nm or
smaller, preferably from 5 nm to 300 nm or so. If the thickness of the
emitter layer is smaller than 10 nm, the diode structure may not be
formed locally. As a consequence, some transistor devices may involve a
problem of reduced performance or a conduction failure, leading to a
reduction in fabrication yield. A thickness greater than 300 nm, on the
other hand, may develop a problem of high fabrication cost and material
cost.

(Collector Layer)

[0065] From an organic semiconductor material, the collector layer in this
embodiment is formed between the base electrode and the collector
electrode. The material usable for the formation of the collector layer
can be an n-type organic semiconductor material or p-type organic
semiconductor material that is commonly employed as an organic
semiconductor material. No particular limitation is imposed on the n-type
organic semiconductor material insofar as it can transport electrons, and
therefore, a common n-type organic semiconductor material can be used.
Likewise, no particular limitation is imposed on the p-type organic
semiconductor material insofar as it can transport holes, and therefore,
a common p-type organic semiconductor material can be used. For example,
the above-described n-type semiconductor material (electron transport
material) or p-type semiconductor material (hole transport material) for
use in the emitter layer can be used. In the collector layer, it is also
possible to use one or more of the materials for forming the
light-emitting layer, which will be described subsequently in the
description of the light-emitting transistor devices according to the
present invention.

[0066] As the material for forming the collector layer in this embodiment,
any desired material can be used without any particular limitation
insofar as it is an n-type organic semiconductor material or p-type
organic semiconductor material. It is desired to form the collector layer
with alkyl-containing PTCDI or fullerene (C60), which is an n-type
organic semiconductor material. The collector layer may use a material
that is commonly employed as a charge transport material. Although an
n-type semiconductor material or p-type semiconductor material can be
used singly, the collector layer may also have a mixed layer structure
formed from two or more n-type semiconductor materials or p-type
semiconductor materials or a stacked structure formed of organic
semiconductor layers of two or more n-type semiconductor materials or
p-type semiconductor materials.

[0067] As a particularly preferred form, it is possible to mention a
stacked collector layer obtainable by forming on the collector layer an
organic semiconductor layer from Me-PTCDI and then forming another
organic semiconductor layer from C60. The organic semiconductor layer of
C60 arranged below the base electrode can form an energy barrier to block
an off-state current.

[0068] When a semiconductor layer formed from Me-PTCDI or fullerene is
used as the collector layer, the surface of the semiconductor layer 21,
on which the base electrode 13 is to be formed, becomes rough because the
alkyl-containing PTCDI or fullerene is an organic compound having high
crystallinity. Therefore, the base electrode 13 arranged on the
semiconductor layer of the organic semiconductor material of high
crystallinity is also provided with a rough surface. The base electrode
having the rough surface includes thinner portions and thicker portions
even when it is formed with a predetermined average thickness. When the
base electrode 13 is provided with such a rough surface, a current
amplifying effect can be stably obtained in particular.

[0069] The thickness of the collector layer in this embodiment may be
generally from 50 nm to 5,000 nm, preferably from 100 nm to 500 nm or so.
A thickness smaller than 50 nm may lead to inoperability as a transistor
device, while a thickness greater than 5,000 nm may result in a problem
of high fabrication cost and material cost. The collector layer may
desirably have a high charge mobility, with at least 0.0001 cm2/Vs
being desired. A low charge mobility develops a problem such as reducing
its performance as a transistor device, for example, lowering its
on-state current.

(Electrodes)

[0070] A description will now be made about the electrodes used in the
transistor device of this embodiment. As the electrodes that make up the
transistor device of this embodiment, there are the collector electrode
11, emitter electrode 12 and base electrode 13. As illustrated in FIG. 1,
the collector electrode 11 is generally arranged on a substrate (not
shown), the base electrode 13 is generally arranged such that it is
embedded between the semiconductor layers 21 and 22, and the emitter
electrode 12 is generally arranged on a side opposite the collector
electrode 11 such that the semiconductor layers 21,22 and the base
electrode 13 are interposed between the emitter electrode 12 and the
collector electrode 11.

[0071] The materials to be used for the electrodes in the transistor
device of this embodiment will next be described. When the collector
layer 21 making up the transistor device of this embodiment is, for
example, an n-type semiconductor layer formed from an organic compound,
examples of a material that forms the collector electrode 11 include
transparent conductive oxides such as ITO (indium tin oxide), indium
oxide, IZO (indium zinc oxide), SnO2 and ZnO, metals having large
work functions such as gold and chromium, and conductive high-molecular
materials such as polyaniline, polyacetylene, polyalkylthiophene
derivatives and polysilane derivatives. Illustrative of a material that
forms the emitter electrode 12 are metals having small work functions,
for example, simple metals such as aluminum and silver, magnesium alloys
such as Mg/Ag, aluminum alloys such as Al/Li, Al/Ca and Al/Mg, alkali
metals led by Li, alkaline earth metals led by Ca, and alloys of these
alkali metals and alkaline earth metals. When the collector layer 21
making up the transistor device of this embodiment is a hole transport
layer formed from an organic compound, on the other hand, the
above-described forming material of the collector electrode 11 and the
above-described forming material of the emitter electrode 12 are
reversed.

[0072] As the base electrode 13 forms a Schottky contact with the material
that makes up the organic semiconductor layer, the materials usable for
the collector electrode 11 and emitter electrode 12 can also be mentioned
as materials for the base electrode. Although not specifically limited,
preferred base electrode materials include metal-containing materials
having small work functions, for example, simple metals such as aluminum
and silver, magnesium alloys such as Mg/Ag, aluminum alloys such as
Al/Li, Al/Ca and Al/Mg, alkali metals led by Li, alkaline earth metals
led by Ca, and compounds of these alkali metals and alkaline earth
metals, such as Li/F. Transparent conductive oxides such as ITO (indium
tin oxide), indium oxide, IZO (indium zinc oxide), SnO2 and ZnO,
metals having large work functions such as gold and chromium, and
conductive high-molecular materials such as polyaniline, polyacetylene,
polyalkylthiophene derivatives and polysilane derivatives can also be
used provided that they can form a Schottky contact with the charge (hole
or electron) injection layer.

[0073] When the collector layer 21 is formed by vacuum deposition of the
above-described organic compound of high crystallinity, the surface of
the collector layer 21, on which the base electrode 13 is to be formed,
is rough as mentioned above. The base electrode 13 arranged on the
crystalline collector layer 21 is also provided with a rough surface. The
base electrode 13 having the rough surface includes thinner portions and
thicker portions even when it is formed with a predetermined average
thickness. According to the present invention, a current amplifying
effect can be stably obtained when the base electrode 13 is provided with
such a rough surface.

[0074] As a base electrode for suppressing a dark current in an off-state
and achieving a high on/off ratio, it is preferred to use a base
electrode with an oxide film formed on an electrode surface by forming an
electrode from aluminum or an aluminum/calcium alloy and then subjecting
it to thermal oxidation treatment in air. Further, the use of a layered
structure formed of an aluminum layer and a lithium fluoride layer as a
base electrode makes it possible to form a transistor device capable of
providing a large on-state current, suppressing a dark current and
achieving a high on/off ratio.

[0075] When it is desired to use the below-described light-emitting
transistor device of the present invention as an organic, light-emitting
transistor device of the bottom emission structure that outputs light
from the side of the substrate, at least the collector electrode 11 may
preferably be formed from a transparent or half-transparent material.
When it is desired to fabricate an organic, light-emitting transistor
device of the top emission structure that outputs light from the side of
the emitter electrode 12, on the other hand, the base electrode 13 and
emitter electrode 12 may preferably be formed from a transparent or
half-transparent material. As the transparent or half-transparent
electrode material capable of providing an improved efficiency of light
output by such a construction, a transparent conductive oxide such as ITO
(indium tin oxide), indium oxide, IZO (indium zinc oxide), SnO2 or
ZnO can be used.

[0076] The thickness of the base electrode 13 employed in the transistor
device of this embodiment may preferably be from 0.5 nm to 100 nm. When
the thickness of the base electrode 13 is 100 nm or smaller, electrons
accelerated at the base voltage Vb are allowed to easily pass. The base
electrode 13 can be used without any problem insofar as it is arranged
without any interruption (without any defect such as a pit or fracture).
A thickness smaller than 0.5 nm, on the other hand, may produce a defect
so that the resulting transistor device may not operate as an organic
transistor device.

[0077] A description will next be made about a process for forming the
electrodes in the transistor device of this embodiment. The collector
electrode 11 and emitter electrode 12 out of the above-described
respective electrodes may preferably be formed by a vacuum process such
as vacuum deposition, sputtering or CVD or a coating process, and their
thicknesses may be, for example, from 10 nm to 1,000 nm or so although
the thicknesses vary depending on the materials to be used. A thickness
smaller than 10 nm may lead to inoperability as a transistor device,
while a thickness greater than 1,000 nm may result in a problem of high
fabrication cost and material cost. On the other hand, the base electrode
13 may also be formed preferably by a vacuum process such as vacuum
deposition, sputtering or CVD or a coating process, and its thickness may
be, for example, from 0.5 nm to 100 nm as described above although the
thickness varies depending on the material to be used.

(Dark-Current Suppression Layer)

[0078] In the transistor device of this embodiment, a dark-current
suppression layer may be formed to suppress a dark current. The
dark-current suppression layer may preferably be formed by subjecting the
base electrode 13 to heat treatment after its formation. Further, a dark
current, which flows through the transistor device of this embodiment
when the voltage Vb is not applied between the emitter electrode 12 and
the base electrode 13, can be effectively suppressed by forming the base
electrode 13 from a metal and forming an oxide film of the metal on one
side or both sides of the base electrode 13.

[0079] Owing to the arrangement of the dark-current suppression layer
between the collector electrode 11 and the base electrode 13 as described
above, it is possible to effectively suppress a dark current from
flowing. As a result, the on/off ratio can be improved further. As
readily appreciated from the foregoing, the transistor device of this
embodiment apparently functions as an efficient current-amplifying
transistor device like a bipolar transistor device, and functions as an
excellent organic transistor device capable of bringing about a high
on/off ratio, a large collector current and a current amplification
factor.

(Light-Emitting Transistor Devices)

[0080] Light-emitting transistor devices according to the present
invention include the light-emitting transistor device A having a
light-emitting layer in the emitter layer 22 of the transistor device
(MBOT) and the light-emitting transistor device B having a light-emitting
layer in the collector layer 21 of the transistor device (MBOT).

[0081] As illustrated in FIG. 2, the light-emitting transistor device A
according to the present invention has an organic EL device portion 31
between a base electrode 13 and an emitter electrode 12 in a transistor
device (MBOT) according to the present invention, and the organic EL
device portion 31 may comprise an organic light-emitting layer and, if
necessary or preferred, one or more layers selected from the group
consisting of a hole injection layer, a hole transport layer, an electron
transport layer and an electron injection layer.

[0082] As illustrated in FIG. 3, on the other hand, the light-emitting
transistor device B according to the present invention has an organic EL
device portion 31 between a base electrode 13 and a collector electrode
11 in a transistor device (MBOT) according to the present invention, and
the organic EL device portion 31 may comprise an organic light-emitting
layer and, if necessary or preferred, one or more layers selected from
the group consisting of a hole injection layer, a hole transport layer,
an electron transport layer and an electron injection layer.

(Light-Emitting Transistor Device A)

[0083] By forming the organic EL device portion 31 in the emitter layer 22
of the transistor device (MBOT) of according to the present invention, an
emission of light occurs as a result of deactivation of excitons formed
by recombination between holes associated with a hole current from the
base electrode 13 and electrons from the emitter electrode. The
transistor device of this embodiment can, therefore, be used as a
light-emitting transistor device A. The light-emitting layer can be used
without any problem insofar as it is formed from a light-emitting
material employed in organic EL devices. The light-emitting layer can be
formed between the p-type organic semiconductor layer and the n-type
organic semiconductor layer. When the n-type organic semiconductor layer
or p-type organic semiconductor layer is formed from a light-emitting,
organic semiconductor material, the organic semiconductor layer can also
serve as the light-emitting layer so that no additional light-emitting
layer needs to be formed. To obtain an improved luminous efficiency, an
electron injection layer and a hole injection layer may be formed.

[0084] As illustrated in FIG. 2, the structure of the light-emitting
transistor device A is characterized in that it has the organic EL device
portion 31 between the base electrode 13 and the emitter electrode 12 and
the organic EL device portion 31 includes the organic light-emitting
layer and, if necessary or preferred, one or more layers selected from
the group consisting of a hole injection layer, a hole transport layer,
an electron transport layer and an electron injection layer. As the
organic EL device portion 31 includes at least one light-emitting layer,
an emission of light by a large current is feasible. No micropatterning
is needed for the base electrode unlike the conventional SIT structure,
large-current modulation is feasible at a low voltage, and further, an
improved on/off ratio is available. The light-emitting transistor device
A can, therefore, be provided as a practical light-emitting transistor
device having a simple structure.

(Light-Emitting Transistor Device B)

[0085] As depicted in FIG. 3, the light-emitting organic transistor device
B according to the present invention has the organic EL device portion 31
between the base electrode 13 and the collector electrode 11, the organic
EL device portion includes the organic light-emitting layer and one or
more layers selected from the group consisting of a hole injection layer,
a hole transport layer, an electron transport layer and an electron
injection layer. The organic light-emitting transistor device B can,
therefore, be provided as a light-emitting transistor device, which
includes the organic EL device portion and its drive transistor in
combination, requires no micropatterning for the base electrode unlike
the conventional SIT structure, can perform large-current modulation, is
high in emission brightness, excellent in contrast and superb in
frequency characteristics owing to a high on/off ratio, and can perform a
self-emission. The light-emitting organic transistor device B can be
fabricated by a vapor deposition process only, can be formed on a
flexible substrate of plastics or the like, and therefore, can be
provided as a light-transmitting transistor device having a small-size,
light-weight and simple structure and high practical utility.

[0086] As illustrated in FIG. 3, the structure of the light-emitting
transistor device B according to the present invention has the organic EL
device portion 31 between the base electrode 13 and the collector
electrode 11, and the organic EL device portion 31 includes at least one
light-emitting layer. The light-emitting transistor device B can,
therefore, perform a surface light emission. No micropatterning is needed
for the base electrode unlike the conventional SIT structure,
large-current modulation is feasible at a low voltage, and an improved
on/off ratio is available. The light-emitting transistor device B can,
therefore, be provided as a light-transmitting transistor device having a
simple structure and high practical utility. As the emitter layer in the
light-emitting transistor device B is an n-type semiconductor layer
formed from an electron transport material, no electron transport layer
is specifically arranged. However, an electron transport layer may be
arranged on the side of the base electrode 13 relative to the organic EL
device portion 31 as needed.

(Organic EL Device Portion)

[0087] An organic EL device portion 31, which is shown in FIG. 4 and is
usable in the light-emitting transistor device A, is provided, in an
ascending order from the side of an electrode as a positive electrode
(from the side of the base electrode), with a hole injection layer 41, a
hole transport layer 42 and a light-emitting layer 43. An exciton
blocking layer 44 is arranged on the side of the emitter electrode 12
(see FIG. 2) relative to the light-emitting layer 43. Below the emitter
electrode 12, an electron transport layer composed of an n-type organic
semiconductor layer 22A (see FIG. 2) is formed. As numerous charges are
accelerated to reach the light-emitting layer 43 of the organic EL device
portion 31 in this light-emitting transistor device A, the injection of
charges from the emitter electrode 12 is easy. The organic EL device
portion 31, therefore, has a merit that it does not necessarily require a
charge injection layer. There is, accordingly, an advantageous effect
that an alkali metal, which facilitates the injection of electrons but is
prone to oxidation, needs not to be used as a negative electrode unlike
conventional organic EL device portions. An organic EL device portion 31
(see FIG. 3) which is usable in the light-emitting transistor device B
has a construction reversed upside down from the construction shown in
FIG. 4, with a need for replacement of "hole" by "electron". Described
specifically, an exciton blocking layer, a light-emitting layer, an
electron transport layer and an electron injection layer are stacked in
this order from the collector electrode 11 toward the base electrode 13
(see FIG. 3). As numerous charges are accelerated to reach the
light-emitting layer of the organic EL device portion 31 in this
light-emitting transistor device B, the injection of electrons from the
collector electrode 11 is easy. The organic EL device portion 31,
therefore, has a merit that it does not necessarily require a charge
injection layer.

[0088] No particular limitation is imposed on the forming material of the
light-emitting layer 43 employed in the organic EL device portion 31 in
the present invention, insofar as it is a material commonly employed as a
light-emitting layer in an organic EL device. Examples include dye-based
light-emitting materials, metal-complex-based light-emitting materials,
and high molecular light-emitting materials.

[0090] Illustrative of the metal-complex-based light-emitting materials
are metal complexes each having Al, Zn, Be or the like or a rare earth
metal such as Tb, Eu or Dy, as a center metal, and an oxadiazole,
thiadiazole, phenylpyridine, phenylbenzoimidazole, quinoline or like
structure as a ligand, such as aluminum-quinolinol complexes,
benzoquinolinol-beryllium complexes, benzoxazole-zinc complexes,
benzothiazole-zinc complexes, azomethyl-zinc complexes, porphyrin-zinc
complexes, and phenanthroline-europium complexes.

[0093] Examples of the forming material of the hole injection layer 41
employed in the organic EL device portion 31 in the present invention
include, in addition to the compounds exemplified above as light-emitting
materials for the light-emitting layer 43, phenylamine derivatives,
starburst amine derivatives, phthalocyanine derivatives, oxides such as
vanadium oxide, molybdenum oxide, ruthenium oxide and aluminum oxide,
amorphous carbon, and derivatives of polyaniline, polythiophene and the
like.

[0094] As a forming material, the hole transport layer 42 employed in the
organic EL device portion 31 in the present invention can use, for
example, the above-described p-type semiconductor material usable for the
organic semiconductor layers. Usable examples include those commonly used
as hole transport materials, such as phthalocyanine, naphthalocyanine,
polyphyrin, oxadiazole, triphenylamine, triazole, imidazole, imidazolone,
pyrazoline, tetrahydroimidazole, hydrazone, stilbene, pentacene,
polythiophene and butadiene, and their derivatives. As the forming
material of the hole transport layer 42, a commercially-available, hole
transport material such as, for example,
poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (abbreviation:
PEDOT/PSS, trade name: "CLEVIOS P" (product of H.C. Starck Clevis GmbH)
can also be used. The hole transport layer 42 can be formed by a vapor
deposition process or by using a coating formulation for the formation of
hole transport layers, which contains such a compound. It is to be noted
that such a hole transport material may be mixed in the
light-transmitting layer 43 or the hole injection layer 41.

[0095] As the hole transport material in the present invention, the
above-described material can be used. In the organic layers such as the
above-mentioned light-emitting layer 43 and charge transport layer, a
light-emitting, oligomer or dendrimer material or a charge transport or
injection, oligomer or dendrimer material may be incorporated as needed.
As the charge mobility of the hole transport layer 42, it is preferred to
have a hole mobility of 10-6 cm2/Vs or higher. Any desired
material other than the above-mentioned materials may also be used
provided that it has higher transport capability for holes than for
electrons. The hole transport layer 42 can be not only a single layer
containing one of the materials having high hole transport capability but
also a stacked layer of two or more layers containing different ones of
such materials, respectively.

[0096] In the present invention, the organic EL device portion 31 may
include an electron injection layer as needed. Examples of the forming
material of the electron injection layer include, in addition to the
compounds exemplified above as light-emitting materials for the
light-emitting layer 43, alkali metals and alkaline earth metals such as
lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium,
calcium, strontium and barium, and halides and oxides of such alkali
metals and alkaline earth metals.

[0097] The exciton blocking layer 44 in the organic EL device portion 31
in the present invention functions as a hole blocking layer, an electron
blocking layer and the like, prevents penetration of carriers (holes and
electrons), allows efficient recombination of carriers. When
bathocuproine (BCP) is used as a forming material for the exciton
blocking layer 44, the exciton blocking layer 44 does not block electrons
from Me-PTC but blocks holes from Alq3 because BCP has a LUMO energy
level substantially equal to that of Me-PTC and a HOMO energy level
higher than that of Alq3. As the excitation state of BCP is higher in
energy than that of Alq3, excitons produced in Alq3 do not diffuse into
BCP.

[0098] The above-described individual layers that make up the organic EL
device portion 31 in the present invention can be formed as films by a
vacuum deposition process, or can be formed by separately dissolving or
dispersing their forming materials in portions of a solvent such as
toluene, chloroform, dichloromethane, tetrahydrofuran or dioxane to
prepare coating formulations and coating or printing these coating
formulations with a coater or printer.

EXAMPLES

[0099] A description will hereinafter be made about examples of the
present invention. Transistor devices fabricated in the examples and a
comparative example were evaluated by the following method.

(Evaluation of Transistor Devices)

[0100] With respect to each transistor device fabricated, a collector
voltage Vc was applied between an emitter electrode and a collector
electrode, and a base voltage (Vb) to be applied between the emitter
electrode and a base electrode was modulated in a range of from 0 V to 3
V. As output modulation characteristics of the transistor device, changes
in base current Ib and collector current Ic (on-state currents, off-state
currents) were measured while applying the base voltage Vb (0 to 3 V)
between the emitter electrode and the base electrode with the collector
voltage (Vc) being kept constant between the emitter electrode and
collector electrode. Further, the ratio of a change in collector current
to a corresponding change in base current, i.e., the current
amplification factor (hFE) and the ratio of an on-stage current to a
corresponding off-state current, i.e., the on/off ratio were calculated.

[0101] As illustrated in FIG. 5, an ITO-coated transparent substrate was
provided as a collector electrode 11. After an organic semiconductor
layer (average thickness: 250 nm) of N,N'-dimethylperylenetetracarboxylic
acid diimide (Me-PTCDI, organic semiconductor material) was formed on the
collector electrode 11, another organic semiconductor layer (average
thickness: 50 nm) of fullerene (C60) was formed to provide a collector
layer 21. Subsequent to the formation of an electrode layer (average
thickness: 3 nm) of lithium fluoride as an electron injection layer, a
base electrode layer (average thickness: 15 nm) of aluminum was stacked
to form a base electrode 13. On the base electrode 13, a p-type organic
semiconductor layer (average thickness: 30 nm) of copper phthalocyanine
and an n-type organic semiconductor layer (average thickness: 50 nm) of
fullerene (C60) were stacked in this order by a vacuum deposition
process, whereby an emitter layer 22 of a diode structure was provided.
An emitter electrode 12 (average thickness: 30 nm) of silver was then
stacked by a vacuum deposition process to obtain a metal-base organic
transistor device (MBOT) of Example 1.

[0102] As output modulation characteristics of the transistor device
obtained in Example 1, changes in collector current Ic and base current
Ib were measured while applying a base voltage Vb (0 to 2 V) between the
emitter electrode and the base electrode and also while applying no base
voltage Vb, both with the collector voltage Vc being kept constant at 3 V
between the emitter electrode and collector electrode. The changes in
collector current while the base voltage was applied (Ic-Vb
characteristics) are shown in FIG. 7. The on-state current and current
amplification factor of the collector current Ic when a collector voltage
(Vc: 3 V) and a base voltage (Vb: 1.1 V) were applied, the off-state
current when Vb=0 V, and the on/off ratio are presented in Table 1.

Example 2

Fabrication of Transistor Device by Diode Structure (C60/Pentacene)

[0103] Similar to Example 1, an ITO-coated transparent substrate was
provided as a collector electrode 11. After an organic semiconductor
layer (average thickness: 250 nm) of N,N'-dimethylperylenetetracarboxylic
acid diimide (Me-PTCDI, organic semiconductor material) was formed on the
collector electrode 11, another organic semiconductor layer (average
thickness: 50 nm) of fullerene (C60) was formed to provide a collector
layer 21. Subsequent to the formation of an electrode layer (average
thickness: 3 nm) of lithium fluoride as an electron injection layer on
the collector layer 21, a base electrode layer (average thickness: 15 nm)
of aluminum was stacked to form a base electrode 13. On the base
electrode 13, a p-type organic semiconductor layer (average thickness: 30
nm) of pentacene and an n-type organic semiconductor layer (average
thickness: 50 nm) of fullerene (C60) were stacked in this order by a
vacuum deposition process, whereby an emitter layer 22 of a diode
structure was provided. An emitter electrode 12 (average thickness: 30
nm) of silver was further stacked by a vacuum deposition process to
obtain a metal-base organic transistor device (MBOT) of Example 2.

[0104] As output modulation characteristics of the transistor device
obtained in Example 2, changes in collector current Ic and base current
Ib were measured while applying a base voltage Vb (0 to 2 V) between the
emitter electrode and the base electrode and also while applying no base
voltage Vb, both with the collector voltage Vc being kept constant at 3 V
between the emitter electrode and collector electrode. The changes in
collector current while the base voltage was applied (Ic-Vb
characteristics) are shown in FIG. 8. The on-state current and current
amplification factor of the collector current Ic when a collector voltage
(Vc: 3 V) and a base voltage (Vb: 1.7 V) were applied, the off-state
current when Vb=0 V, and the on/off ratio are presented in Table 2.

Example 3

Fabrication of Light-Emitting Transistor Device by Diode Structure

(B4PYMPM/Pentacene)

[0105] As depicted in FIG. 6, an ITO-coated transparent substrate was
provided as a collector electrode 11. After an organic semiconductor
layer (average thickness: 250 nm) of N,N'-dimethylperylenetetracarboxylic
acid diimide (Me-PTCDI, organic semiconductor material) was formed on the
collector electrode 11, another organic semiconductor layer (average
thickness: 50 nm) of fullerene (C60) was formed to provide a collector
layer 21 of a stacked structure. Subsequent to the formation of an
electrode layer (average thickness: 3 nm) of lithium fluoride as an
electron injection layer on the collector layer 21, an electrode layer
(average thickness: 15 nm) of aluminum was stacked to form a base
electrode 13. On the base electrode 13, a p-type organic semiconductor
layer (average thickness: 30 nm) 22B of pentacene, a hole transport layer
(average film thickness: 10 nm) of α-NPD
[Bis(N-(1-naphthyl)-N-phenyl)benzidine] as a pore transport material, an
organic light-emitting layer 43 (average film thickness: 20 nm) of Alq3
[tris(8-hydroxyquinolinol)aluminum) complex], an electron transport layer
22A (average thickness: 10 nm) of a pyrimidine derivative B4PYMPM
(below-described formula (1)) as an electron transport material, and
lithium fluoride as an electron injection layer (average film thickness:
0.5 nm) were successively stacked by a vacuum deposition process, whereby
an emitter layer 22 of a diode structure was formed with the organic
light-emitting layer 43 embedded therein. On the emitter layer 22, an
emitter electrode 12 (average thickness: 100 nm) of aluminum was further
stacked by a vacuum deposition process to obtain a light-emitting,
metal-base, organic transistor device (MBOT) of Example 3.

##STR00001##

[0106] A collector voltage (Vc: 12 V) was applied to the resultant
light-emitting transistor device to confirm if any emission of light
would occur. No emission of light was confirmed when no base voltage (Vb:
0 V) was applied, but an emission of light from the emitter layer was
confirmed when a base voltage (Vb: 10 V) was applied.

Comparative Example 1

Fabrication of Transistor Device from C60

[0107] An ITO-coated transparent substrate was provided as a collector
electrode, and a collector layer was formed as in Example 1. Subsequent
to the formation of an electrode layer (average thickness: 3 nm) of
lithium fluoride as an electron injection layer on the collector layer as
in Example 1, a base electrode layer (average thickness: 15 nm) of
aluminum was stacked to form a base electrode. On the base electrode, an
n-type organic semiconductor layer (average thickness: 80 nm) of
fullerene (C60) was stacked by a vacuum deposition process, whereby a
non-diode structure of the n-type organic semiconductor material was
formed. An emitter electrode 12 (average thickness: 30 nm) of silver was
further stacked by a vacuum deposition process to obtain a transistor
device of Comparative Example 1.

(Evaluation Results)

[0108] As output modulation characteristics of the resultant transistor
device, changes in collector current Ic and base current Ib were measured
while applying a base voltage Vb (0 to 2 V) between the emitter electrode
and the base electrode and also while applying no base voltage Vb, both
with the collector voltage Vc being kept constant at 3 V between the
emitter electrode and collector electrode. The transistor device was
confirmed to operate as MBOT. The evaluation results are presented in
Tables 1 and 2, and are shown in FIGS. 7 and 8.

[0109] The organic transistor device of Example 1 was confirmed to
function as a current-amplifying transistor device. It shows the large
amplification factor of 98.4 at the low voltage. Further, its on-state
current was 36.4 mA/cm2 (Vb: 1.1 V) and 145 mA/cm2 (Vb: 2.0 V).
It, therefore, performed modulation of a very large current as a
transistor device. In addition, its on/off ratio (Vb: 1.1 V) was 20.3.

[0110] The organic transistor device of Example 2 was also confirmed to
function as a current-amplifying transistor device. It shows the large
amplification factor of 160.4 at the low voltage. Further, its on-state
current was 42.5 mA/cm2 (Vb: 1.7 V) and 62 mA/cm2 (Vb: 2.0 V).
It, therefore, performed modulation of a very large current as a
transistor device. In addition, its on/off ratio (Vb: 1.7 V) was 116.8.

[0111] On the other hand, the transistor device of Comparative Example 1
was confirmed to operate and to function as a transistor device that as
MBOT, has current amplification capability, and reduces a dark current.
When the collector voltage (Vc: 3 V) was applied, however, the on-state
current value was 2.2 mA/cm2 (Vb: 1.1 V) and 7.1 mA/cm2 (Vb:
1.7 V) so that only a small current was available.

[0112] The transistor devices of Examples 1 and 2 provided, as their
on-state currents, currents approximately 17 and 6 times as large as
their corresponding on-currents available from the transistor device of
Comparative Example 1, and further, their off-state currents were
suppressed. The transistor devices of Examples 1 and 2, therefore, showed
excellent transistor characteristics. Further, the light-emitting
transistor device of Example 3 was confirmed to show a high on/off ratio.

[0113] As demonstrated above, the transistor device according to the
present invention has been confirmed to perform large-current modulation
at a low voltage, to provide a high on/off ratio, and to apparently
function as a similar effective current-amplifying transistor device as a
bipolar transistor. Further, the introduction of a light-emitting layer
in the emitter layer has been found to provide a light-emitting
transistor device which can perform a self-emission while enjoying the
advantageous effects of the transistor device according to the present
invention.

INDUSTRIAL APPLICABILITY

[0114] The transistor device according to the present invention provides a
small off-state current and a high on/off ratio, and therefore, can be
used as a drive device for a display such as an organic EL display. When
an organic light-emitting layer is incorporated, the transistor device
according to the present invention can be used as an organic
light-emitting transistor device.